Come ottenere quanti byte ha caricato finora?

Question

Il mio metodo di upload come di seguito:

private void upload(String Server, String FilePath, String SavePath, String NewName) {
String end = "\r\n";
String twoHyphens = "--";
String boundary = "*****";
try {
URL url = new URL(ActionUrl);
HttpURLConnection con = (HttpURLConnection) url.openConnection();
con.setDoInput(true);
con.setDoOutput(true);
con.setUseCaches(false);
con.setRequestMethod("POST");
con.setRequestProperty("Connection", "Keep-Alive");
con.setRequestProperty("Accept", "text/*");
con.setRequestProperty("Content-Type", "multipart/form-data; boundary=" + boundary);
ds = new DataOutputStream(con.getOutputStream());
ds.writeBytes(twoHyphens + boundary + end);
ds.writeBytes("Content-Disposition: form-data;" + "name=\"folder\"" + end + end);
ds.write(SavePath.getBytes("UTF-8"));
ds.writeBytes(end);
ds.writeBytes(twoHyphens + boundary + end);
ds.writeBytes("Content-Disposition: form-data;" + "name=\"Filedata\"; filename=\"");
ds.write(NewName.getBytes("UTF-8"));
ds.writeBytes("\"" + end);
ds.writeBytes(end);

FileInputStream fStream = new FileInputStream(uploadFile);
int bufferSize = 1024;
byte[] buffer = new byte[bufferSize];
int length = -1;
while((length = fStream.read(buffer)) != -1) {
ds.write(buffer, 0, length);
}       
ds.writeBytes(end);
ds.writeBytes(twoHyphens + boundary + twoHyphens + end);

fStream.close();
ds.flush();
InputStream is = con.getInputStream();
int ch;
StringBuffer b = new StringBuffer();
while((ch = is.read()) != -1) {
b.append((char)ch);
}
System.out.println("UPLOAD" + "SUCCESS");
ds.close();
}
catch(Exception e) {
e.printStackTrace();
}
}

Voglio mostrare la barra di avanzamento. Ma non sapevo quanto byte sia stato caricato finora. Qualche metodo per ottenere il numero di byte che è stato caricato?